Continuously variable transmissions (CVT) are commonly used on a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts and scooters. They comprise a driving pulley connected to a motor, a driven pulley connected to wheels or a track, and a trapezoidal belt transmitting torque between the driving pulley and the driven pulley. The CVT automatically changes the ratio as required by load and speed conditions, providing an increased torque under high loads at low speed and yet controlling the rotation speed of the motor as the vehicle accelerates. A CVT may be used with all kinds of motors, such as internal combustion engines or electric motors.
The sides of the trapezoidal belt are, on each pulley, gripped between two opposite flanges that are coaxially mounted around a main shaft. Generally, one flange, called xe2x80x9cfixed flangexe2x80x9d, is rigidly connected to one end of the shaft. The other flange, called xe2x80x9cmovable flangexe2x80x9d, is free to rotate and slide with reference to a portion of the shaft. At low speed, the winding diameter of the driving pulley is small and the winding diameter of the driven pulley is maximum. As the rotation speed of the driving pulley increases, the movable flange of the driving pulley gets closer to the fixed flange and thus forces the trapezoidal belt to wind on a greater diameter. Since the length of the trapezoidal belt does not significantly changes, the trapezoidal belt exerts a radial force towards the center of the flanges of the driven pulley in addition to the tangential driving force. This radial force constrains the driven pulley to have a smaller winding diameter. Therefore, the movable flange of the driven pulley moves away from the fixed flange until the return force exerted by a spring, usually a torsion spring, counterbalances the radial force exerted by the trapezoidal belt.
Yet, change in the load may also produce a variation of the winding diameter of the driven pulley. More particularly, a greater load induces a greater winding diameter thereof and vice versa. This is caused by a cam system comprises a cam plate having a plurality of symmetrically-disposed cam surfaces. Corresponding cam followers are in engagement with the cam surfaces. The cam followers are usually slider buttons or rollers. The cam plate or the set of cam followers is rigidly connected at the back side of the movable flange and the other of them is rigidly connected to the shaft. The closing effect of the cam system on the belt tension is then somewhat proportional to output torque. The belt tension is high under high loads at low speed, thereby preventing belt slippage. However, it is lower at. high speed to avoid excessive pressure of the belt against the flanges of the pulleys and to maintain a good efficiency. When the rotation speed of the motor decreases, the winding diameter of the driving pulley decreases and the radial force exerted by the trapezoidal belt decreases, thus allowing the driven pulley to have a greater winding diameter as the torsion spring moves the movable flange towards the fixed flange. An example of such a variable speed transmission is disclosed in U.S. Pat. No. 3,286,330.
Many vehicles include a rearward rotation mode, whereby the driving torque from the motor is provided for movement in a direction opposite the normal driving direction. The inverted driving torque is obtained by an appropriate gear train or, on some vehicles, by inverting the rotation of the motor. One drawback of a conventional driven pulley is that: it does not provide an efficient operation under a rearward rotation mode, usually because the cam followers are then no longer in engagement with their respective cam surface. Yet, the torque from the motor is then opposite the torsion spring. As a result, the ratio of the transmission is likely to be initially higher that required, which then unnecessarily increases the load on the motor.
A second and similar drawback occurs under motor braking phases, such as when the vehicle is decelerating or traveling down a hill. A conventional driven pulley is difficult to control under those phases. Generally, the rotation speed of the motor a then becomes essentially a function of the traveling speed of the vehicle, transforming the CVT into substantially a one-speed transmission. This is especially a problem for an electric motor with regenerative capabilities and in which a large portion of the inertia of the vehicle is ought to be transformed back into electrical energy and stored in the batteries. Since the rotation speed varies greatly, the output voltage from the motor also greatly fluctuates under motor braking phases and thus requires the intensive use of a sophisticated voltage regulator.
Further, an electric motor with regenerative capabilities is typically 25% less efficient when used. as a generator than it is when used as a motor. To compensate, the rotation speed of the motor should be higher during the regeneration than it is when the motor is used to drive the vehicle.
In JP-A63009767, there is disclosed a torque cam device designed to prevent the slippage between a single set of cam rollers and two opposite annular cam plates. The device comprises a support for supporting the cam rollers between the respective cam surfaces of the annular cam plates. The support is maintained in a central position between both cam plates by means of compression springs. The device is capable of working under a reverse torque condition but the ratio will tend to increase when such condition occurs.
In JP-A-63214567, there is disclosed a torque cam device used to. prevent the occurrence of an excessive thrust force during normal drive operation. A first cam surface is formed in one among the input and output members and a second cam member so that they are inclined in parallel with each other. Rollers are clamped between both cam surfaces by a compression spring.
In U.S. Pat. No. 4,523,917, there is disclosed a driving pulley provided with two sets of cams which face each other. The angle of the cams is different between the two sets. This pulley uses one of the sets of cams to generate a large gripping force in function of the torque between the minimum and an intermediary ratio. Then, between the intermediary ratio and the maximum ratio, the second set of cams is used. The cams of the second set have a steeper angle so that a smaller gripping force in generated in function of the torque.
The object of the present invention is to provide an improved driven pulley which can suitably and efficiently operate in both forward and rearward directions to regulate the rotation speed, thereby resolving the above-identified drawbacks.
More particularly, the present invention relates to a driven pulley for use in a continuously variable transmission, the driven pulley being coaxially mountable around a main shaft and comprising:
a first flange having a conical wall on one side thereof;
a second flange coaxial with the first flange and having a conical wall which faces the conical wall of the first flange to form a belt-receiving groove in which a belt is to be partially wound, the second flange being at least axially movable with reference to the first flange;
a first annular cam plate comprising at least two inclined first cam surfaces that are substantially identical and symmetrically-disposed thereon;
a set of at least two first cam followers, each first cam follower being in engagement with a respective one of the first cam surfaces;
a first radial support coaxial with the first and the second flange, the first support being axially movable and pivotable with reference to the first( flange;
a second cam plate comprising at least two inclined second cam surfaces, substantially identical and symmetrically-disposed;
a second radial support coaxial with the first and the second flange, the second support being rigidly connectable to the main shaft;
a first spring set between the second flange and the first support; and
a second spring set between the first support and the second support;
the driven pulley being characterized in that:
the second cam surfaces of the second annular cam plate have an inverted inclination with reference to the first cam surfaces; and
the driven pulley further comprises a set of at least two second cam followers, each second cam follower being in engagement with a respective one of the second cam surfaces;
whereby one among the first cam plate and the set of first cam followers is mounted on the second flange and the other is mounted on the first support, and whereby one among the second cam plate and the set of second cam followers is mounted on the first support and the other is mounted on the second radial support.
A non restrictive description of the preferred embodiments is given hereinafter with reference to the appended figures.